This disclosure generally relates to systems and methods for measuring the polarization of light in images. In particular, this disclosure relates to the use polarization and polarimetry for visually detecting objects of interest. As used herein, the term “polarimetry” means the measurement and interpretation of the polarization of transverse waves, such as electromagnetic waves.
The general problem addressed herein is to improve systems and methods for measuring the polarization of light in images, specifically images captured by moving vehicles. Polarization and polarimetry can help users detect many objects of interest. For example, in a natural scene full of unpolarized light, smooth surfaces appear as linearly polarized light; those smooth surfaces often correspond to artifacts such as weapons or downed aircraft (which are frequently sought by the military) or foreign object damage (which most vehicle operators try to avoid). Polarization lets surveillance analysts' vision penetrate haze or the glare from surfaces like water or windows. Polarization enables military analysts to find submarines and mines or spot snipers hiding behind windows, and enables fishermen to find schools of fish. Polarization may also help civilian users measure weather parameters or assess the health of forests and crops.
Despite these benefits, polarimetry and polarized imagery are rarely used. The reasons are cost, weight, and reliability. In general, it is not sufficient to put a single polarizing filter in front of a single camera. To measure polarization in an image and discern which parts of the image have different polarization than other parts, one must capture an image with at least two and usually three orientations of a polarizing filter. In prior art, this has meant: (1) an electrically controlled rotating filter mounted to a camera lens, (2) an electrically controlled filter wheel with several polarizing filters mounted at different angles, or (3) multiple cameras, each with a differently oriented polarizing filter. The cost, weight, and reliability penalties of these approaches have precluded most uses of polarimetry for images taken outside a laboratory.
In the case of an electrically controlled rotating filter mounted to a camera lens, a filter wheel is configured to position polarizing filters with three or four different orientations in front of a single camera. A filter wheel is a fairly robust optical component with moving parts. It is about as heavy as a small camera used on a typical unmanned aerial vehicle (UAV). It occupies substantial volume. Having an electromechanical actuator, it is substantially less reliable than a digital camera and therefore reduces the reliability of an aircraft mission system.
A rotating polarizer in front of a single camera is smaller than a filter wheel, but is still a robust optical component with moving parts. It substantially increases the weight of a small camera and may substantially increase its volume. It contains an electromechanical actuator, which reduces the reliability of an aircraft mission system.
In the third case, a system comprising multiple cameras facing the same direction, each with a differently oriented polarizer in front of it, imposes a small penalty to cost, weight, and reliability for each camera. However, using three or four cameras instead of one increases cost and weight and decreases reliability of the system.
In accordance with a further development, differently oriented polarizing filters are placed in front of various pixels in a charge-coupled device (CCD). Such a camera would produce a digital image structured like a three- or four-color picture, but each “color” would correspond to the intensity of a different polarization. It is not clear that a pixel-by-pixel polarizing filter can be made economically. The camera does not allow actual color imaging (e.g., red, blue, and green) concurrent with the polarimetry. One such CCD chip is designed to output four “colors” (one for each polarization) rather than the usual three expected by image file formats. This poses technical and economic barriers to widespread acceptance.
It would be desirable to provide improved means and methods for collecting visual polarimetry data from a moving vehicle (e.g., an aerial vehicle) with an optimal combination of low cost, low weight, and low reliability penalty.